A PRE-IMPACT MODEL FOR IONIC AND ATOMIC DESORPTION INDUCED BY FAST MULTICHARGED PROJECTILES

Desorption induced by highly charged particles is described through a model based on direct Coulomb interaction between the projectile and a dsorbed molecules. It is assumed that the emission of ionized and neut ral species from metals or insulators is triggered by the electric fie ld of the projectile before its impact on the surface. The projectile interaction with the adsorbate layer induces ionizations or electronic excitations which lead to subsequent desorption through processes suc h as Coulomb explosion or molecular dissociation, causing the secondar y emission of ions or neutral species. The pre-impact desorption (PID) model proposed here is based on a potential-ejection model normally u sed for projectile velocities below the Bohr velocity. It is considere d that surface ionization occurs in a well defined period of time, whe n over-the-barrier electron emission (or electron promotion) is possib le. The model predicts a q(3)/cos theta behavior of the desorption yie ld, q being the projectile charge and theta the angle of incidence. Th e q(3)-dependence has been observed for H+ emission induced by keV and MeV energy beams. It also predicts an increase of the induced desorpt ion yield, as the ionization potential of the adsorbed layer decreases . With regard to the yield dependence on the projectile velocity, the pre-impact model predicts a maximum around the Bohr velocity; the dera iled description of the projectile-surface interaction may affect the position of this maximum but not the general trend of the function.